1. Field of the Invention
The present invention relates generally to a hydraulic control apparatus for a hydraulically operated automatic transmission of an automotive vehicle, and more particularly to improvements in such a hydraulic control apparatus, for controlling a shifting operation of a frictional coupling device of the transmission with a reduced shifting shock, and with increased fail-safe reliability.
2. Discussion of the Prior Art
An automatic transmission for a motor vehicle usually incorporates a frictional coupling device such as a clutch for shifting from a neutral position to a forward drive position (drive position, 1st-speed position, 2nd-speed position, etc.), or vice versa, in response to an operation of a shift lever. A commonly used hydraulic control apparatus for controlling engaging and disengaging operations of such a frictional coupling device employs two fixed flow restrictors, for slowly effecting the engaging and disengaging operations of the frictional coupling device to shift the automatic transmission between the neutral and forward drive positions. One of the flow restrictors is provided with a flow checking ball which prevents a fluid flow in a direction toward the frictional coupling device, so that the rate of flow of the fluid into the coupling device is lower than that of the fluid flow from the coupling device. Thus, the engaging action of the coupling device takes place comparatively slowly.
However, the rate of fluid flow from the frictional coupling device, or the disengaging speed of the coupling device cannot be regulated or restricted. To reduce the shifting shock upon shifting of the transmission to the neutral position, the flow restrictors should have a relatively small cross sectional area of fluid flow. Accordingly, it takes a considerably long time after the shifting to the neutral position is commanded (after the fluid discharge from the coupling device is started), and before the disengaging action of the coupling device is started. Namely, this time lag cannot be reduced without increasing the shifting shock.
In view of the above drawback, a hydraulic control apparatus as shown in FIG. 10 is proposed. This control apparatus is adapted to control a frictional coupling device 6, and includes a shift lever valve 2 operated in response to an operation of a shift lever, and an accumulator 4. When a line pressure PL is produced in a fluid passage 8 by the shift lever valve 2, the line pressure PL is applied to the frictional coupling device 6, through fluid passages 8A and 8B which communicate with each other, and through fluid passages 8C and 8D which also communicate with each other. Therefore, the fluid is supplied to the frictional coupling device 6 at a comparatively high rate. When a piston 4A of the accumulator 4 is moved to a lower position as seen in FIG. 10, the fluid passages 8C and 8D are disconnected, whereby the fluid flows toward the coupling device 6, through the fluid passages 8A and 8B only. Accordingly, the rate of flow of the fluid into the coupling device 6 is lowered, and the coupling device is engaged at a relatively low speed. When the piston of the accumulator 4 is moved to the lowest position, the fluid passages 8C and 8D are again brought into communication with each other, whereby the frictional coupling device 6 is stably maintained in its fully engaged state.
Although the above proposed hydraulic control apparatus is capable of changing the rate of fluid flow between the shift lever valve 2 and the coupling device 6, the rates of fluid flows into and from the coupling device 6 cannot be controlled, independently of each other. That is, the engaging and disengaging operations of the coupling device 6 cannot be controlled independently of each other. Therefore, the degree of freedom to provide a compromise between the shifting speed and the shifting shock is relatively low, or the engaging and disengaging actions cannot be optimally controlled to satisfy the required operating characteristics.
A technique associated with the above problem is disclosed in laid-open Publication No. 48-38574 of examined Japanese Patent Application published for opposition purpose. The disclosed technique lies in the use of a controller for electrically controlling the rate of fluid flow into a frictional coupling device or the rate of increase in the pressure in the coupling device, when the transmission is shifted between the neutral position N and the reverse position R, and between the parking position P and the reverse position R.
When the above technique is applied to the frictional coupling device for shifting the transmission between the neutral position N and the forward drive position D (S, L), the transmission may be shifted to the forward drive position or to the neutral position in the event of some electrical trouble with the controller. Therefore, it is practically difficult or impossible to apply the above technique to the frictional coupling device for shifting the transmission between the neutral and forward drive positions.